Certain types of radars that exploit the Doppler effect for target detection are called Doppler radars. The Doppler effect manifests itself when there is a relative range rate, or radial velocity, between the radar and the target. When the radar's transmit signal is reflected from such a target, the carrier frequency of the return signal will be shifted. Assuming a collocated transmitter and receiver, the resulting Doppler frequency shift is a function of the carrier wavelength and the relative radial velocity (range rate) between the radar and the target. When the target is moving away from the radar, the relative radial velocity, or range rate, is defined to be positive and results in a negative Doppler shift.
Doppler radar systems can be either continuous wave (CW) or pulsed. CW radars simply observe the Doppler shift between the carrier frequency of the return signal relative to the transmit signal.
Pulse Doppler radars use a coherent train of pulses where there is a fixed or deterministic phase relationship of the carrier frequency between each successive radio frequency (RF) pulse. Coherence concentrates the energy in the frequency spectrum of the pulse train around distinct spectral lines, separated by the pulse repetition frequency (PRF). This separation into spectral lines allows for discrimination of Doppler shifts.
The pulsed nature of the transmitted signal permit time gating of the receiver, which allows for blanking of direct transmit energy leakage into the receiver. This permits the use of a single antenna for transmit and receive, which otherwise would not be feasible.
Pulse Doppler radars can also use range gating, which divides the inter-pulse period into cells or range gates. The duration of each range gate is typically less than or equal to the inverse of the transmit pulse bandwidth. Range gating can help eliminate excess receiver noise from interfering with target return pulses, and allow range measurement with pulse delay ranging (i.e., measuring the time between transmission of a pulse and reception of the target echo).